1. Field of the Invention (Technical Field)
The present invention relates to methods for non-destructively evaluating the condition and remaining service life of ceramic coatings.
2. Description of Related Art
Thermal barrier coatings (TBCs) are used in gas power and aircraft engine turbines to protect metal vanes and blades from the combustion gas stream. TBCs increase the useful life of turbine components and improve turbine efficiency by allowing increased operating temperatures. TBCs are typically used in the first turbine stage directly following the combustor and provide insulation and oxidation resistance for the metal components. These coatings consist of four basic layers: the substrate, the bond coat, a thermally grown oxide layer (TGO) which develops during use, and the ceramic top coat. During the course of part service, which includes cyclic heating and stress, TBC integrity degrades. Degradation modalities include microcracking within the top coat and shearing or rumpling at the interface between the ceramic top coat and bond layer where the TGO resides. Ultimately, degradation of the TBC leads to failure. If this condition occurs while the part is in service, the entire engine can be compromised. Optimization of part service life while maintaining an adequate safety factor relies on non-destructive evaluation (NDE) methods for integrity assessment.
A variety of optical approaches have been used as NDE strategies to inspect ceramic coating condition during manufacturing or use. Key parameters for assessment have included layer thickness, degree of defect, or stress present in a sample, but no single embodiment has provided a comprehensive assessment of multiple health parameters.
Assessment of the TBC thickness is critical at the point of manufacture to maintain quality control. U.S. Pat. No. 8,300,232, to Sansom et al., describes a two-color infrared wavelength approach for range detection throughout the thickness of a TBC. Longer wavelengths of light penetrate preferentially through the highly scattering ceramic TBC, and strategic selection of two wavelengths provides strong backscattered radiation at the air/top coat and top coat/bond layer interfaces. While this method captures the coating thickness, it is unable to monitor changes to coating integrity.
For part inspection, imaging solutions are preferred to spot inspection in order to increase monitoring areas and reduce inspection time. Multiple imaging techniques have been employed for detection of defects and cracks. U.S. Pat. No. 5,426,506, to Ellingson et al., stipulates the use of polarized light to detect both surface and near-subsurface defects, though no correlation is made between the presence of cracks and fatigue of part in service. J. I. Eldridge, et al., “Monitoring Delamination Progression in Thermal Barrier Coatings by Mid-Infrared Reflectance Imaging”, International Journal of Applied Ceramic Technology 3, 94-104 (2006), utilizing long penetrating mid-infrared imaging wavelengths of 3-6 μm, demonstrated that the intensity of backscattered reflectance from a TBC increases with thermal cycling. This imaging approach is limited to two-dimensions and cannot distinguish the location of crack-related scattering as a function of depth within a coating. For three-dimensional resolution, W. A. Ellingson et al., “Optical NDE Methods for Ceramic Thermal Barrier Coatings,” Mater. Eval. 64, 45-51 (2006), used optical coherence tomography (OCT) to inspect TBCs. However, the wavelength of light limited the depth of imaging into the coating. Nor did Ellingson et al. correlate features or metrics derived from OCT images to aging or remaining service life of the coating.
Stress measurement has been viewed as a highly accurate gauge of TBC health. Stress levels within a TBC have been correlated to shifts in fluorescence monitored by photoluminescence piezo spectroscopy (PLPS). J. A. Nychka, et al., “Damage quantification in TBCs by photo-stimulated luminescence spectroscopy”, Surf. Coat. Technol. 146, 110-116 (2001). U.S. Pat. No. 7,918,141, to Sathish et al., describes a method for measuring local residual stress of an outer surface portion of a TBC. While these approaches map stress near the TBC surface or at the TGO interface, they are not able to detect the presence of cracking throughout the thickness of a TBC nor the roughness of the TBC substrate interfacial region.
A holistic approach to TBC health inspection would require simultaneous assessment of cracking and degree of stress. A. B. Vakhtin, et al., paper presented at the ASME Turbo Expo 2007: Power for Land, Sea and Air (GT2007), Montreal, Canada, 2007, combined OCT imaging of TBCs with measurements of stress within the TBC using PLPS. PLPS, however, represents a laboratory-based method and is not ideally suited to ready inspection of in-service parts. An all-optical imaging method for health inspection is essential.